The present invention generally relates to pattern inspection and foreign substance inspection technology for detecting defects (such as short circuit and disconnection) and foreign objects on a pattern being examined, and particularly to a defect inspection method for examining defects and foreign objects on a pattern such as semiconductor wafer, liquid crystal display and photomask, and to a defect inspection apparatus using that method. In the following description, it is assumed that the defect includes a foreign object.
There is known an inspection apparatus of this kind. As disclosed in JP-A-7-318326 (prior art 1), while a pattern to be examined is being moved, an imager such as line sensor is used to detect the image of the pattern, and the detected image signal and an image signal a certain time delayed from that signal are compared in their brightness so that a disagreement therebetween can be recognized as a defect. In addition, another example is disclosed in JP-A-8-320294 (prior art 2).
According to the prior art 2, when a pattern to be inspected is a semiconductor wafer of which the chip areas each have, in a mixed state, high-density pattern regions such as memory mats, and low-density pattern regions such as peripheral circuits, the detected analog pattern image signal is converted to a digital image signal, and further converted to a signal of gradations so that the high-density and low-density regions have a certain brightness or contrast ratio decided from the brightness frequency distribution of the detected image, and this gradation image signal is aligned with and compared with a separate gradation image signal so that fine defects can be examined with high accuracy.
In the recent LSI production, the circuit pattern formed on the wafer has been developed to a very fine pattern such as a line width of 0.25 xcexcm or below in accordance with the needs for higher integration. This pattern width is the resolution limit of the image-forming optical system. Therefore, the image-forming optical system is advancing toward use of high NA (numerical aperture) and ultra-high optical resolution technology.
However, the high NA has reached the physical critical limit. Therefore, the wavelength of light to be used for the detection should be reduced to ultraviolet light (UV light or DUV light) region as an essential approach.
Moreover, since the inspection is required to be fast conducted, the method of scanning on a sample by fine laser beam cannot be used. If the laser beam is spread out up to full field of view in order to illuminate at a time, speckles occur, and overshoot and undershoot called ringing are caused at the edges of the circuit pattern, thus degrading the picture quality.
The present invention, in order to solve the above problems, is to provide a method and apparatus for fast examining a fine circuit pattern with high resolution to detect defects thereon.
According to the invention, a laser source is used as a light source, and means for suppressing the laser speckle from occurring is provided in the light path so that coherency-reduced light is irradiated on the object surface to examine the object image.
According to the present invention, as the means for suppressing the laser speckle from occurring, means is provided to gather rays of light from the light source at a point or a plurality of points on the pupil of an objective lens and to scan those points on the pupil in timing with the accumulation time of a detector.
Moreover, in order to improve the pattern contrast, considering that the polarized state of laser can be freely controlled, the orientation of the polarization of illuminating light and ellipticity are controlled so that the polarized component as part of detected light can be detected.
In addition, a plurality of laser beams are used as light sources, because it is possible to expect not only the increase of the defect detection sensitivity, but also other various effects such as long life and countermeasure against breakdown. Also, laser beams of different wavelengths are used and combined because they are effective for controlling the polarized state. Since each laser output can also be reduced, the life of the laser sources can be extended. The addition of laser beams is made by use of a polarization beam splitter, dichroic mirror or half mirror.
The beams of the same wavelength can be processed, by a polarization beam splitter, to be laser beams of which the polarization directions are perpendicular to each other. The dichroic mirror is able to change the polarization directions of laser beams of different wavelengths to parallel or orthogonal directions. As compared with the half mirror, those processes can be achieved with high efficiency. In addition, the polarized state of one or both of different-wavelength beams can be changed by use of a wave plate.
According to the invention, a laser source for ultraviolet (UV) laser beam is used. Here, UV light and DUV are generally called UV light.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.